(a) Field of the Invention
The present invention relates to an apparatus and method for controlling the opening and/or closing operation of a liquid valve dispensing liquid fed thereto from a reservoir, using the inherent gas pressure from the reservoir.
The present application particularly relates to a control system for a pressurized liquid gas reservoir incorporated in an expendable refrigerant refrigeration system. In such instance, the invention provides a control system which utilizes gas from the reservoir to control the flow of liquid from the reservoir in a series of cyclic injections.
(b) Description of Prior Art
The use of reservoir gas as a control media is well known as are cyclic control sequences used to achieve "pulsed" refrigerant injections. Applicant's U.S. Pat. No. 4,041,725 dated Aug. 16, 1977 discloses one method which makes use of two pressure actuated three-way valves, connected by gas lines damped by restrictors, such that the output of one addresses the actuation of the other to create a rythmic assymetrical oscillation which results in one "pulsed" injection of refrigerant for each full cycle of the control system.
The aforementioned prior art invention, in practical application, has proved to have but limited facility to vary the ratios of "on" to "off" timing. It has been found for instance, the adjustment of one restrictor affects both the "on" and the "off" durations and a normal maximum differential between the two cannot safely exceed a 5:1 ratio. Nor can the overall duration of a complete cycle be extended beyond about two minutes, without incurring the risk of hysteresis in the valve with consequential failure of the control function.
Working at or near the aforementioned limits of adjustment, requires a delicate, skilled, touch. Field calibration by unskilled operators is deemed all but impossible.
Although the control disclosed in U.S. Pat. No. 4,041,725 has a proven value when used in series with a temperature controller, by providing an extended response time so as to allow the temperature sensor a greater time to react to changed conditions and so avoid an overshoot (of refrigerant injected), it is deemed to have little practical worth as a selectable metering means.
A second problem and one common to most if not all controls used with expendable refrigeration systems, arises whenever the control set point is set, for whatever reason, below the existing cargo temperature. In these circumstances, the control, in seeking to reduce temperatures to that set point and needing to reduce a large cargo mass to achieve that purpose, will commence and continue to inject refrigerant at its maximum rated capacity. Generally, this action will rapidly exhaust the available refrigerant leaving the product without means of refrigeration for the balance of the journey.